1. Field of the Invention
This invention relates to plastic containers to which a lid may be peelably sealed.
2. The Prior Art
The art has under consideration containers for the packaging of oxygen-sensitive foods intended for storage under non-refrigerated conditions. These containers are formed from multilayer sheets constructed of an inner barrier layer, juxtaposed adhesive layers on both sides of the barrier layer and outer layers of a polyolefin. For example, British Pat. No. 1,379,106 discloses a five-layer sheet comprised of an inner gas barrier layer formed of a copolymer of vinylidene chloride and vinyl chloride, an ethylene/vinyl acetate copolymer adhesive layer on either side of the barrier layer and an outer layer of a polyolefin such as polyethylene, polypropylene or ethylene/propylene copolymer. The multilayer sheets are disclosed as being ideally suited for making containers of a variety of forms for products that are sensitive to oxygen.
The multilayer sheets of the prior art are conventionally made in a single process step by coextruding two or more polyolefinic layers and intermediate layers comprised of an adhesive tie layer material into a continuous sheet to form a unitary structure. In the case of multilayer gas barrier sheets, an inner layer of a gas barrier material is coextruded with the polyolefinic and adhesive layers.
A multilayer sheet structure having gas barrier properties which has been found particularly suitable for the manufacture of plastic containers used for the packaging of oxygen sensitive food products is constructed of an inner gas barrier layer such as ethylene vinyl alcohol (EVOH) or vinylidene chloride polymer (SARAN), first and second adhesive tie layers applied to each side of the barrier layer, a first outer layer of low or high density polyethylene, bonded to the first adhesive layer and a propylene polymer or styrene polymer layer bonded to the second adhesive layer.
Containers thermoformed from such multilayer gas barrier sheet are utilized in the packaging of oxygen-sensitive food products which are intended for storage under non-refrigerated conditions. In such application, the containers are formed using a sterile multilayer gas barrier film, and while in a still heated state from the forming station are filled with sterile food product in a filling station inside a sterile enclosure. The filled containers are then heat sealed without it being possible for their contents to be contaminated by a non-sterile atmosphere with a lid sheet whereof at least the side of the lid facing the inside of the containers is composed of a heat activatable thermoplastic material which is sterile.
Multilayer sheets of the non-barrier and gas barrier type are converted to containers using conventional thermoplastic forming techniques such as thermoforming. In thermoforming, the multilayer sheet material is thermoformed by feeding the sheet from roll stock past a heating station where the sheet is heated, usually by passage through an oven or overhead radiant heaters or a combination of both and the heated sheet passes directly to a thermoforming machine which forms the container. Any suitable thermoforming technique such as vacuum forming, pressure forming, plug assist or mechanical forming or any combination of such techniques can be used in the forming of the containers but in all cases the sheet material is preheated generally to temperature of about 375.degree. F. to about 425.degree. F. to effect a degree of softening of the multilayer sheet so that it can be easily thermoformed into containers having uniform sidewalls of the desired thickness.
In recent years the food packaging industry has shown intense interest in plastic containers formed from multilayer sheet of the types described above which can be hermetically sealed with a lid which is readily peelable from the container without tearing or rupturing of the packaging materials. Thus as a simple matter of convenience, it is desirable that the sealed container be openable by manually pulling the lid from the container instead of relying on a knife, scissors or tear tab to open the package.
To effect peelability it is normally required that an intermediate, peelable layer be placed between the container surface and the opposed lid surface. One of the disadvantages encountered in such peel systems is that upon the removal of the lid from the container surface, the phenomenon of "legging" occurs, i.e., multiple strands of the peel layer form between the lid and container surface as the lid is peeled from the container. Legging is unattractive and there exists the possibility of the strand material's falling into and contaminating the food product. Also, this application of a peel layer to the lid or container surface is a costly manufacturing procedure. It would therefore be highly advantageous if the peel layer were an integral part of the container to which a lid could then be directly hermetically and peelably sealed.